Windows often include a vent connected to a frame by a hinge. This arrangement enables the window to be opened widely. However, it is often desirable to be able to restrict the extent to which this type of window can be opened during normal use for reasons of safety and security. Any device for restricting the opening of a window during normal use must also allow the window to be opened fully when required, for example for cleaning or in the case of a fire.
Although a number of restricting devices have been suggested in the prior art, many of such devices are complicated to manufacture and difficult to use. In particular, many of the prior art restricting devices have to be incorporated when a vent is manufactured or hung, and cannot be added to restrict the movement of a vent that is already in place.